FIG. 7 is a block diagram illustrating a configuration of an angle diversity receiving device 600 related to the invention. Referring to FIG. 7, the angle diversity receiving device 600 includes a group of antennas 611 composed of two horn antennas 61A and 61B. The angle diversity receiving device 600 receives a received signal from the horn antenna 61A through a band pass filter (BPF) 221, a low noise amplifier (LNA) 231, a received frequency converter (down converter, D/C) 241, an automatic gain controller (AGC) 251, and an adaptive matched filter (AMF) 131.
Further, the angle diversity receiving device 600 receives a received signal from the horn antenna 61B through a band pass filter 222, a low noise amplifier 232, a received frequency converter 242, an automatic gain controller 252, and an adaptive matched filter 132. In this way, the angle diversity receiving device 600 receives the received signals from the horn antennas 61A and 61B, as two branches (receiving system) in which angle diversity is performed.
The band pass filters (BPF) 221 and 222 allow only signals within a frequency band required for reception to pass. The low noise amplifiers (LNA) 231 and 232 amplify the received signals. The received frequency converters (down converter, D/C) 241 and 242 frequency-convert the received signals. The automatic gain controllers (AGC) 251 and 252 amplify the received signals so as to suppress amplitude change of the received signals outputted to following steps. The adaptive matched filters 131 and 132 decrease unnecessary signals included in the received signal of each branch.
Incidentally, diversity synthesizing circuit 140 performs diversity combining of signals inputted from the adaptive matched filters 131 and 132. An automatic equalization circuit (decision feedback equalizer, DFE) 150 performs automatic equalization on the signals performed the diversity combining and reproduces the received signals.
Such kinds of angle diversity receiving devices are disclosed for example in PTLs (Patent Literatures) 1 to 6 and NPL (Non Patent Literature) 1, except than above described one.
FIG. 8 is a diagram explaining tropospheric scatter propagation. A receiving antenna 812 corresponds to, for example, the group of antennas 611 of the angle diversity receiving device 600 shown in FIG. 7. A radio wave transmitted from a transmitting station 811 is scattered at a first scattering point STa and a second scattering point STb, and received, as signals of orientations DTa and DTb, by the group of antennas 812.
FIG. 9 is a diagram explaining operations of the angle diversity receiving device 600 based on a related technology. The angle diversity receiving device 600 shown in FIG. 7 and FIG. 9 operates as follows. As shown in FIG. 9, two beams of antenna DR6a and DR6b are fixed at predetermined angles. Then, the group of antennas 611 performs angle diversity processing on received signals arriving from the dual beams of DR6a and DR6b. 
The angle diversity receiving device 600 receives an arriving radio wave in a transmitting direction DTa of FIG. 8 based on the beam of DR6a in FIG. 9, and receives an arriving radio wave in a transmitting direction DTb of FIG. 8 based on the beam of DR6b. Here, if a condition of tropospheric scatter changes and a spatial position of the scattering points STa or STb changes, the transmitting direction DTa or the transmitting direction DTb changes. As a result, intensity of the received signals in the angle diversity receiving device 600 may remarkably decrease.
That is, in the angle diversity system in which a direction of the horn antenna is fixed, when an azimuth of an arriving radio wave changes as a result of propagation condition changes, a reception level may be reduced. Particularly, in tropospheric scatter propagation, a condition of scatter propagation in troposphere widely changes due to climate change. Consequently, in the angle diversity system in which the orientations of the antennas are fixed, long-period fading with propagation loss of 10 dB to 20-odd dB occurs throughout one year, and a reception level of the radio wave may decrease.
Additionally, in the angle diversity receiving device, an angle between a plurality of receiving horns is preliminarily arranged so that a correlation value between branches does not increase. In the angle diversity receiving device, a correlation value between the branches may increase as a condition of troposphere scatter changes and a diversity effect may decrease.